which is caused by the spirochete Borrelia burgdorferi, is the most common tick-transmitted illness in the United States. If untreated, Lyme disease can lead to a wide array of complications typically involving the heart, joints, or nervous system. It is widely believed that the motility of B. burgdorferi is essential for the pathogenesis of Lyme disease. B. burgdorferi swims by rotating helical filaments (flagella) that reside in the periplasmic space (the space between the outer membrane and the cell wall material). The rotation of these periplasmic flagella against the cell wall leads to deformations of the cell cylinder, and these deformations exert force against the external environment. The bacterium transitions between the arthropod vector (Ixodid ticks) and mammalian host. This enzootic cycle requires the bacterium to interact with extremely different environments. For example, spirochetes must be able to colonize the tick midgut, and then migrate out of the midgut into the hemocoel. Once in the hemocoel, the bacterium must navigate towards the salivary glands, attach to the acinar surface, penetrate the basal lamina, and enter the salivary ducts. B. burgdorferi is then inoculated into the skin of its mammalian host where it must translocate through the extracellular matrix in order to access small vessels which provide portals for dissemination through the blood. To invade joints and other host tissue, the cells must adhere to the endothelium of blood vessels in target organs and penetrate through them. The unique motility and morphology of B. burgdorferi are presumed to drive many of these processes and are, therefore, considered to be major factors in the pathogenesis of Lyme disease. The principal hypothesis of this proposal is that the internal mechanism driving the motility of B. burgdorferi (i.e., flagellar rotation) is largely unchanged when the spirochete moves between the tick and the mammalian host, but its strategy for motility is substantially different due to differences in the interactions with the different host tissues. This reasoning suggests that the shape; physical parameters, such as the stiffness of the flagella and cell cylinder; and the internal mechanism driving motility have evolved to allow for directed migration in these diverse environments. Therefore, this research will first experimentally test the predictions of a mathematical model developed by the PI that describes the shape and motility of B. burgdorferi using antibiotic-treated cells and genetic manipulations to alter the stiffnesses of the cell wall and flagella. Next, the motility of B. burgdorferi will be examined in gelatin matrices, in order to quantify motility through a controllable model system that mimics the ECM. Finally, modeling and time-lapse fluorescence microscopy will be used to determine the mechanisms of motility in epithelial cell layers, and in the tick and mouse. These aims are directed toward moving the current understanding of motility in non-physiological liquid and/or methycellulose solutions to biologically realistic environments in which spirochetes adhere to cells or ECM in order to complete their enzootic cycle and accomplish their parasitic strategy. PUBLIC HEALTH RELEVANCE: The research described in this proposal will determine the biophysical mechanisms that are involved in the transmission to and invasion of the host that occurs in Lyme disease. Specifically, a quantitative model of the pathogen-host interactions during the progression of Lyme disease will be developed and experimentally tested, which will provide a detailed understanding of the infection process and may lead to novel therapeutic methods.